1. Field of the Invention
The present invention relates to a focusing method for a surveying instrument having a sighting telescope with an AF (automatic focusing) function, an apparatus for carrying out the focusing method, and an arrangement of an AF beam splitting optical system incorporated therein.
2. Description of the Related Art
In general, a surveying instrument, such as an optical distance measuring meter or a total station, etc., is provided with a sighting telescope which is kept focused on a target (object to be surveyed). During surveying, the sighting telescope is directed at the target, and a focusing knob is subsequently manually rotated to focus the telescope on the target. However, it is difficult for an operator to manually perform a focus adjustment while concentrating on the sighting operation. Moreover, the manual focus adjustment using the focusing knob is time consuming.
To solve these problems, surveying instruments equipped with a sighting telescope having an automatic focusing (AF) function have been recently proposed. In the AF optical system, object light transmitted through a focusing lens of the sighting telescope is made incident upon a surface (plane) conjugate with a focal surface (plane), so that the focus state on the conjugate surface is detected to calculate the amount of defocus (defocus amount) of the focusing lens. Consequently, the focusing lens is moved to an in-focus position in accordance with the calculated defocus amount. The principle of the AF operation is per se known and has been widely used in AF single lens reflex cameras.
While a surveying instrument having an AF function is useful, in practice a number of drawbacks exist. Namely, the AF operation is carried out for the target in a focus area within the field of view of the sighting telescope.
Accordingly, if there is an extraneous object e.g., a fence or leaves swaying in the wind, etc., between the sighting telescope and the target, the target can not be correctly focused or "hunting" occurs. Moreover, in a conventional surveying instrument having an AF function, the light path of the sighting optical system is split to produce a focus detecting beam which is detected by an AF sensor provided in the split light path, thus resulting in an increase in the size of the surveying instrument.